Why do you have to play two rounds in the Champions League semi-final?

This is UEFA's decision to increase the number of matches so that participating teams can get more benefits. In the knockout stage of the European Champions League, except for the championship battle, which is a neutral venue, all the other two rounds of knockout are home and away. In addition, there are only two awards in the Champions League, the champion and the runner-up, and there is no competition for the third place.